Substituted piperidines have found use in the treatment of many nervous system disorders. For example, methylphenidate has been used to treat Attention Deficit Disorder (ADD), Attention Deficit Hyperactivity Disorder (ADHD) and cognitive decline in Acquired Immunodeficiency Syndrome (AIDS) and AIDS Related Complex (ARC) patients. (See, e.g., Greenhill, Child & Adol. Psych. Clin. NA., 1995, 4, 123, and Brown, Intl. J Psychl. Med., 1995, 25, 21).
Many currently available synthetic routes to methylphenidate and other substituted piperidines involve preparation of racemic mixtures. (See, e.g., U.S. Pat. No. 2,507,631, to Hartmann, et al., and U.S. Pat. No. 2,957,880, to Rometsch, et al.). There are, however, a number of disadvantages associated with racemic mixtures of such drugs. Current administration of racemic methylphenidate often results in notable side effects such as anorexia, weight loss, insomnia, dizziness and dysphoria. Additionally, racemic methylphenidate produces a euphoric effect when administered intravenously or through inhalation, and thus carries a high potential for substance abuse in patients.
U.S. Pat. No. Nos. 2,507,631 and 2,957,880 disclose synthetic procedures wherein methylphenidate, alternatively known as methyl .alpha.-piperid-2-ylphenylacetate, is prepared through a multi-step process in which 2-chloropyridine and phenylacetonitrile initially are coupled to form .alpha.-pyrid-2-ylphenylacetonitrile. The resulting .alpha.-pyrid-2-ylphenylacetonitrile then is hydrated in the presence of acid to yield .alpha.-pyrid-2-ylphenylacetamide which, in turn, is either: (a) catalytically hydrogenated to yield .alpha.-piperid-2-ylphenylacetamide and then converted to methyl .alpha.-piperid-2-ylphenylacetate, or (b) converted to methyl .alpha.-pyrid-2-ylphenylacetate which, in turn, is hydrogenated to yield methyl .alpha.-piperid-2-ylphenylacetate.
In the first embodiment of U.S. Pat. No. 2,507,631 and in the process described in U.S. Pat. No. No. 2,957,880, .alpha.-piperid-2-ylphenylacetamide is first separated into the threo and erythro diastereomeric racemates. This is accomplished through evaporation of the solvent utilized in the hydrogenation (i.e., acetic acid), addition of sodium hydroxide to precipitate the .alpha.-piperid-2-ylphenylacetamide free base, recrystallization of this amide from ethyl acetate, and preferential crystallization of the erythro form by passing gaseous hydrogen chloride through an ethanolic solution of the amide.
The isolated erythro racemate then is resolved through formation of the l-tartrate salt, repeated recrystallizations of this salt from 96% ethanol are performed until a constant rotation is obtained, and the l-erythro form of .alpha.-piperid-2-ylphenylacetamide is precipitated with sodium hydroxide. The l-erythro form of .alpha.-piperid-2-ylphenylacetamide thus obtained is said to be subjected to epimerization to yield the desired d-threo diastereomer of .alpha.-piperid-2-ylphenylacetamide through treatment with 6M potassium hydroxide. According to the disclosed procedure, the .alpha.-piperid-2-ylphenylacetamide thus obtained is converted to d-threo methyl .alpha.-piperid-2-ylphenylacetate through hydrolysis and esterification.
Some in the art have raised doubts as to whether the procedures disclosed in U.S. Pat. Nos. 2,507,631 and 2,957,880 do, in fact, produce the desired d-threo isomer. Indeed, J. R. Soares, "Stereochemical Studies On Potential Central Nervous System Active Agents and Studies On The Chemistry Of Some 3-Benzoylpiperidines," 1971, Columbia University Ph.D. dissertation, p. 115, discloses that "all attempts to epimerize the resolved erythro-amides to the corresponding threo-amides by the procedure outlined in U.S. Pat. No. 2,957,880! failed completely."
In any event, the synthetic procedure described in U.S. Pat. Nos. 2,507,631 and 2,957,880 is wasteful in that it involves discarding the threo .alpha.-piperid-2-ylphenylacetamide racemate which is isolated following the recrystallization step and which typically represents approximately 25% of the acetamide product obtained via hydrogenation.
Consequently, there remains a need in the art for alternative synthetic procedures for the preparation of methylphenidate and other substituted piperidines. In particular, there is a need for synthetic procedures that do not require separating and discarding threo stereoisomers from the hydrogenation reaction product.